WO2007066005A2

WO2007066005A2 - Method for modifying silicic crystalline materials

Info

Publication number: WO2007066005A2
Application number: PCT/FR2006/002664
Authority: WO
Inventors: Daniel Joubert
Original assignee: Daniel Joubert
Priority date: 2005-12-06
Filing date: 2006-12-06
Publication date: 2007-06-14
Also published as: EP2007484A2; FR2894166A1; FR2894166B1; WO2007066005B1; WO2007066005A3

Abstract

The invention relates to a method for modifying silicic crystalline materials consisting i) in bringing a silicic crystalline material into contact with the aqueous solution of alkali metal silicate also comprising an ion complexing additive and ii) in evaporating water. A composite obtainable by said method and the use of the aqueous solution of alkali metal silicate for inerting the silicic crystalline materials, in particular asbestos are also disclosed.

Description

PROCESS FOR MODIFYING CRYSTALLINE MATERIALS

SILICICS

The present invention relates to a method for modifying silicic crystalline materials, in particular asbestos, as well as the composite material obtained.

Silica crystalline materials such as asbestos have been used extensively as a fire-retardant and insulating material, especially in buildings.

However, exposure to asbestos fibers and dust is dangerous for human health. The harmful properties of asbestos are linked in particular to its fibrous structure and its crystalline character.

Inhaling asbestos particles can cause asbestosis, a disease of the lungs. This disease can transform, after a latency period of thirty years or more, into different cancers, in particular lung cancer and mesothelioma, a form of inoperable cancer of the chest and abdominal wall.

These risks have led to a ban on the use of asbestos. It also appeared necessary to proceed with asbestos removal from public buildings and / or for residential use containing asbestos.

Asbestos removal, carried out mechanically, is accompanied by a significant production of asbestos dust, requiring adequate protection of workers, but also, according to the applicable regulations, the inerting of asbestos products resulting from this operation, before their controlled landfill.

Various processes are known for inerting asbestos.

It is thus possible to subject the asbestos to a heat treatment in order to destroy its crystalline character. However, because of the refractory nature of asbestos, these heat treatments must be carried out at high temperatures, of the order of 1500 ° C. These processes therefore have a significant cost because of their energy consumption. Chemical inerting procedures for asbestos have also been proposed.

It is thus known from US Pat. No. 5,516,973 to attack asbestos with fluorides or hydrofluoric acid. These reagents dissolve the cation components of asbestos, in particular magnesium oxides, as well as the silicic skeleton by forming water-soluble fluoro-silicates. However, the reagents used in this process require very expensive specific equipment.

Finally, it has been proposed, in particular in application WO 9203235, to coat the asbestos in a hydraulic binder. However, this method has the drawback of leading to waste of significant weight and volume which must then be buried.

The object of the present invention is to propose a method for modifying silicic crystalline materials and in particular asbestos which does not have the drawbacks of the prior art, and in particular inexpensive and resulting in space-saving waste.

This object is achieved according to the invention by bringing the silicic crystalline material into contact with a composition having a very strong physicochemical affinity for the various elements present in the structure of this material, which has the consequence of reducing the character dangerous lens of these materials. A stable amorphous / vitreous composite is then obtained and having a notable mechanical resistance, thus preventing the material from crumbling.

More specifically, according to a first aspect, the invention relates to a process for modifying silicic crystalline materials, comprising the steps consisting in:

i) bringing the silicic crystalline material into contact with an aqueous solution of alkali metal silicate further comprising an additive complexing ions; and

ii) evaporate the water. The term “ion complexing additive” is understood to mean a compound, preferably organic, having complexing properties of ions, in particular metal ions.

The term “silicic crystalline material” means a material having an essentially crystalline structure and not amorphous and composed at least in part of silicon oxide tetrahedra.

Particularly targeted are fibrous silicic crystalline materials and in particular asbestos.

In fact, the process described is intended in particular for the treatment of asbestos fibers from asbestos removal operations, before landfilling.

Asbestos is made up of an agglomerate of extremely fine elementary fibrils with specific physical and chemical properties: resistance to high temperatures, incombustibility, high mechanical tensile strength, resistance to chemical attack and to microorganisms, high electrical resistance, flexibility, ease of being spun and woven.

There are six varieties of asbestos, which can be grouped into two main families of mineral silicates according to their crystal structure: amphiboles and serpentines. The chrysotile or white asbestos, of the serpentine class, represents approximately 95% of the world asbestos production. The risks posed by chrysotiles, which some present as safe compared to other varieties of asbestos, are the subject of controversy. Amphiboles include five minerals, two of which are used industrially: amosite or brown asbestos and crocidolite or blue asbestos.

Apart from the application to asbestos, the process is however also interesting for the inerting of other dangerous silicic crystalline materials. Table 1: Varieties and asbestos species (From Kirk -Othmer / Encyclopedia of Chemical Technology (Vol 3) 3 ^rd edition)

* Asbestos species of commercial importance

Such materials are for example fine sands and all divided forms of crystalline silica such as quartz, but especially cristobalite and tridymite which have fibrous textures.

Indeed, these finely divided products, having a particle size of 0.5 to 100 microns, can, like asbestos, cause diseases like silicosis when they are inhaled and therefore present in the same way a danger for human health. .

The proposed process comprises as essential step the contacting of the silica crystalline material to be treated with an aqueous solution of alkali silicate further comprising an additive complexing ions. The additives are preferably organic and miscible with the silicate. They are intended to complement and accelerate the interaction of the alkali silicate solution with said silica crystalline material.

The solutions of alkali silicates have very specific properties which have earned these solutions the name "soluble glass".

These solutions actually contain rendered silica oligomers ionic and mobile thanks to the alkali metal, for example the sodium present (Si-ONa bond breaking the silica network) and also to hydroxyls resulting from hydrolysis with water (Si-OH bonds). Their dehydration at temperatures between 100 and 200 ^° C. leads to a return to a vitreous solid state which is practically insoluble if the dehydration is advanced.

Solutions of alkaline silicates, in particular sodium silicates, have a very strong physical and chemical affinity for silicate solid surfaces and interfaces, such as clays, natural or synthetic zeolites, precipitation silica or crystalline silica, kaolins. . Asbestos, silicate products, are among the products for which silicates in solution have this very strong physicochemical affinity, which leads to the formation of addition compounds having amorphous structures not defined in the scientific literature relating to these subjects.

This high affinity can be troublesome, for example in laundry washing formulations where it is desired to formulate together sodium silicate and synthetic 4A zeolites ion exchangers.

Indeed, we observe that if we mix and dry together, without care, silicate and a crystalline zeolite, we obtain an amalgam of the two products, in which the zeolite has lost its ion exchange properties, and this in an almost irreversible way .

On the other hand, in the context of inerting of silicic crystalline materials such as asbestos, this property is very advantageous and can be used to obtain, under mild conditions, a stable composite between the crystalline material rendered totally or partially amorphous. and alkali silicate.

Preferably, the alkali silicate is a sodium or potassium silicate.

The SiO ₂ / Me ₂ O molar ratio characterizing the alkali metal silicate (Me = alkali metal) is generally between 1 and 4, preferably between 2 and 3.5.

In principle, it is preferred that the alkali silicate solution is concentrated. This limits the amount of water to be evaporated in step ii). The term “concentrated solution” is understood to mean a solution with a concentration of at least 20%, preferably from 30 to 60%, and more particularly from 30 to 50% of dry extract of alkaline silicate.

According to the invention, the alkali silicate solution also comprises an ion complexing agent.

Indeed, the materials to be treated contain metallic species, in particular of magnesium or iron and the additive contributes to the destructuring of the crystal by trapping these species inserted in the meshes of the crystal structure.

The amount of complexing additive is not particularly limited and depends in particular on that of metallic species present in the material to be treated. Generally, the alkali silicate solution may contain

0.1 to 20%, preferably 0.1 to 10% by weight of additive in the form of an aqueous solution of concentration between 25 and 50% by weight.

According to a preferred embodiment, the complexing agent is an amphiphilic compound. It therefore has a surfactant nature, which makes it possible to improve the wetting of the silicic crystalline material.

Obtaining a homogeneous and stable composition on storage is particularly advantageous. Also, a complexing agent is preferably chosen which makes it possible to obtain a homogeneous solution, and very particularly a single-phase composition.

Preferably, the ion complexing additive makes it possible in particular to complex the metal species, in particular magnesium and iron. Such additives can be chosen from amino-phosphonate organic complexing agents, and more particularly from ethylene diamino sodium tetraphosphonate, sodium diethylenetriaminopentaphosphonate known under the brand names Briquest 423 and Briquest 543 (marketed by Rhodia, France).

According to another preferred embodiment, the ion complexing additive is an amphoteric or zwitterionic compound.

These complexing additives are advantageously also compatible with a concentrated alkali silicate solution.

According to a preferred embodiment, the ion complexing additive is a compound of general formula:

wherein R is a group selected from alkyl, alkenyl, alkyne, aryl, alkylaryl or arylalkyl C ₈ to C _8.

Such products are commercially available, for example under the name Mirataine H2CHA or JCHA (Rhodia, France).

In this variant, the ion complexing additive is both amphiphilic and amphoteric.

In this case, the additives are preferably chosen so that the two additives are miscible with the silicate in a proportion of

0.1 to 20% by weight for the total of the two additives.

Step i) can be carried out by any means known in the art allowing complete wetting of the material with the solution. A simple means is the spraying of the liquid composition onto the crystalline material. Alternatively, the material to be treated can be brought into contact by immersion or soaking in the alkali silicate solution.

The quantity of inerting solution used is not critical, as long as it is sufficient to completely coat the material to be treated. As an indication, it is possible to add 10 to 150%, preferably 30 to 100% by weight of alkali silicate solution, expressed as dry product, relative to the weight of material to be treated.

The time for bringing the material to be treated into contact with the inerting solution is chosen so as to ensure good impregnation of the material to be treated. Generally, a few minutes are sufficient.

Step ii) can be carried out by drying at a moderate temperature between 20 and 110 ^° C., optionally under the effect of ventilation. Preferably, it is carried out at room temperature, without any particular energy contribution.

But of course, in order to speed up the obtaining of a dry compound, and to return to a glassy state of sodium silicate, it is possible to subject the compound obtained to a temperature much higher than those set out above. , without limitation. At higher treatment temperatures, from 450 - 600 ^° C., it can be seen that the inerting solution also plays a role as a flux.

The advantage of this process compared to existing systems is notably that:

the reagents are aqueous, slightly alkaline, not dangerous for the user and the environment;

it can be implemented at room temperature;

- it is generally inexpensive; and

it does not generate by-products to be treated.

The process according to the invention can be applied to the inerting of silicic crystalline materials such as asbestos in various ways.

Advantageously, it allows inerting of the asbestos in place, and therefore avoiding the deblocking operation, generating dust.

As a variant, the asbestos treated on site according to the method described can be subsequently removed, but under better conditions, practically free of dust.

Finally, of course, the treatment can be applied to asbestos deflocculated in an appropriate treatment unit, before setting dump.

Surprisingly, the composite obtained after evaporation of water resists water and has a mechanical resistance appreciably superior to the initial silica crystalline material. In particular, it has a high rigidity, is solid and free of very fine dust or microparticles.

Thus, the invention therefore relates, according to a third aspect, to a composite of crystalline material and of silicate capable of being obtained by the process described.

Finally, according to a last aspect, the invention relates to the use of an alkali metal silicate solution as described above for the inerting of crystalline silicic materials, such as in particular asbestos.

The invention will be described in more detail in the nonlimiting examples below, and with regard to the single figure which shows:

Fig. 1: a photograph of an untreated asbestos sample

(left) and a sample of inert asbestos with sodium silicate according to Example 2 (right).

Fig. 2: X-ray spectrum of an untreated asbestos sample;

Fig. 3: X-ray spectrum of an asbestos sample treated with an alkali silicate solution; and

Fig. 4: X-ray spectrum of an asbestos sample treated with an alkaline silicate solution comprising an ion complexing agent;

EXAMPLE 1

We take a 3.5g piece of asbestos braid used as insulation. It is noted at the time of cutting that a clearly visible amount of fine fibrous particles is produced.

This specimen is immersed in a silicate solution having an Rm SiO ₂ / Na ₂ O molar ratio of 3.4, containing 35% by weight of dry matter.

After 5 minutes of contact time, the piece of braid impregnated with silicate is placed on a metal grid to be drained. After draining, the piece of braid wet with silicate is weighed and a mass of 8.6 g is noted. Asbestos in braided form therefore absorbed 5.1 g of silicate solution.

The specimen moistened with silicate is then left for 6 hours in the open air, at approximately 25 ^° C., and then weighed again. There is a mass of about 5.7g. The asbestos therefore absorbed 2.2 g of partially dehydrated silicate. The dry silicate / asbestos treatment rate is therefore 63%.

The composite obtained is in a stiffened, solid form, free of all forms of very fine dust or microparticles. Microscopic examination confirms the disappearance of fibrillated forms of asbestos.

EXAMPLE 2

The procedure is the same as in Example 1, except that the silicate-impregnated sample is dried for 2 hours at 120 ^° C.

The initial mass of asbestos braid is 2.6g. The drained impregnated braid weighs 4.7 g. Asbestos therefore absorbed 2.1 g of silicate solution. The braid after impregnation and drying weighs 3.5g. The dry silicate / asbestos treatment rate is therefore 35%.

It is noted that the drying operation at 120 ° C. made it possible to dehydrate the silicate solution impregnated on the asbestos almost completely.

The material obtained has an amorphous amalgamated appearance, and is extremely rigid and resistant to breakage. As can be seen in the single figure, it has no visible fibrils. EXAMPLE 3

The procedure is the same as in Example 1 and 2, but replacing the silicate of Rm 3.4 with a silicate of Rm SiO ₂ / Na ₂ O = 2.

Two composite test pieces are therefore obtained after drying for 6 hours at room temperature and drying for 2 hours at 120 ^° C., respectively.

The aspects of these two test pieces are very close to those of Examples 1 and 2. In particular, they do not have any visible fibrils. EXAMPLE 4

Example 1 is reproduced, but by replacing the treatment solution with a mixture composed of 90% by weight of silicate solution Rm 3.4 to 35% of dry extract and of 10% of dimethylene sodium triaminopentaphosphonate in solution at 40% (Briquest 543 from Rhodia).

The drying is carried out at room temperature for 3 hours.

The final treatment rate is 40%, expressed in dry silicate + Briquest solution relative to the mass of asbestos.

The composite obtained is mechanically resistant, rigid, and without any apparent fibrils.

EXAMPLE 5

95 parts by weight of a 45% aqueous solution of dry extract of alkaline silicate with a molar ratio SiO ₂ ZNa ₂ O = 2 are mixed with 5 parts by weight of an amphoteric complexing agent (Mirataine H2CHA, Rhodia,

France). The mixture is homogeneous and monophasic, it is ready to use and stable over time.

A 5.2 g piece of chrysotile asbestos braid is impregnated with an equal weight of the mixture as prepared, then it is left to dry for 24 hours. The solid composite obtained has a glassy appearance and amorphous. It is weighed (7.8 g) and is therefore composed of 5.2 g of asbestos coated with 2.6 g of "dried" silicate.

After 72 hours, an analysis of the crystallinity of the composite is carried out. To this end, the following samples are subjected to X-ray diffraction:

- a piece of untreated asbestos braid (reference);

- a piece of asbestos braid treated as described above, but with an alkaline silicate not added with amphoteric complexing agent; and

- a piece of asbestos braid treated as in Example 5.

The X-ray spectrum on the reference asbestos sample (Figure 2) shows the characteristic peaks of chrysotile asbestos.

The asbestos sample treated with the alkali silicate solution alone (Figure 3) has an amorphous and / or vitreous mass appearance. However, we observe on the X-ray spectrum, in addition to the black baseline corresponds to an amorphous and / or glassy mass, always intense peaks corresponding to the characteristic lines of crystalline asbestos.

The asbestos sample treated with a mixture of alkali silicate solution and amphoteric complexing agent has a similar appearance, in which the asbestos is well coated in a mass of amorphous silicate. However, it can also be seen that the characteristic lines of crystalline asbestos have lost in intensity, which makes it possible to estimate that the sample has lost more than 70% of its crystalline structure.

The loss of the crystalline character of asbestos is a primary issue, since this is what gives it its dangerous nature. Also, a composite in which asbestos has largely lost its crystallinity is relatively harmless and can be deposited in an uncontrolled landfill, without risk.

The proposed method makes it possible, by treatment with an aqueous alkali silicate solution comprising an ion complexing agent, to achieve such a composite.

Claims

1. Method for modifying silicic crystalline materials, comprising the steps consisting in:

i) bringing the silicic crystalline material into contact with an aqueous alkali metal silicate solution comprising an ion complexing additive; and

ii) evaporate the water.

2. The method of claim 1, wherein the silicic crystalline material is fibrous.

3. Method according to claim 1 or 2, wherein the silica crystalline material is asbestos.

4. Method according to claim 1 or 2, wherein the silica crystalline material is fine silica sand.

5. Method according to one of claims 1 to 4, wherein the alkali metal silicate is sodium silicate.

6. Method according to one of claims 1 to 5, wherein step i) is carried out by spraying or by dipping.

7. Method according to one of claims 1 to 6, wherein the aqueous alkali metal silicate solution comprises a proportion of

0.1 to 10% by weight of ion complexing additive.

8. Method according to one of claims 1 to 7, wherein the alkali metal silicate solution comprising the ion complexing additive is monophasic.

9. Method according to one of claims 1 to 8, wherein the ion complexing additive is an amphoteric compound.

10. Method according to one of claims 1 to 9, in which the ion complexing additive is a compound of general formula:

wherein R can be a group selected from alkyl, alkenyl, alkyne, aryl, alkylaryl or arylalkyl C ₈ to C _8.

11. Method according to one of claims 1 to 10, in which the ion complexing additive is sodium diethylene triaminopentaphosphonate.

12. Method according to one of claims 1 to 11, wherein the ion complexing additive is amphiphilic and amphoteric.

13. The method of claim 12, wherein the two additives are miscible with the silicate in an amount of 0.1 to 20% by weight for the total of the two additives.

14. Method according to one of claims 1 to 13, wherein step ii) is carried out at a temperature of 20 to 110 ^° C.

15. Composite of crystalline material and silicate capable of being obtained by the process according to claims 1 to 14.

16. Use of an alkali metal silicate solution comprising an ion complexing additive for inerting silica crystalline materials.